The present invention relates to a phase-shift distributed-feedback semiconductor laser.
Along with the development of various new optoelectronic products, semiconductor laser devices are used as stable light sources in a variety of applications. In applications such as coherent optical systems and photosensor systems, semiconductor lasers having a narrow spectral line width are required. A demand also has arisen for improving FM modulation characteristics when semiconductor lasers are used as light sources for coherent optical communication.
One of the conventional apparatuses characterized by a narrow spectral width is an apparatus constituting a cavity using an external mirror (E. Patzak, A. Sugimura, S. Saito, T. Mukai, and H. Olexen, Electron. Lett. Vol. 19, P. 1026 (1983); and T. P. Lee et. al. Electron. Lett., Vol. 21, P. 1048 (1985)). Since the external mirror is not integrated on a semiconductor chip, the apparatus is not mechanically and thermally stable.
Another conventional example is a phase-shift distributed-feedback semiconductor laser, the main part of which is shown in FIGS. 1A and 1B (H. Haus and C. Shank, IEEE, J. Quantum Electron., Vol. QE-12, P. 532 (1976)). The properties of this semiconductor laser will be described with reference to FIGS. 1A and 1B.
Reference numeral 1 denotes an active layer for performing optical amplification; 2 and 3, cladding layers for confining light; and 5, a diffraction grating formed at a boundary between the cladding layers 1 and 3 to feed back light in a distributed manner.
The structure shown in FIG. 1A belongs to a distributed-feedback semiconductor laser (DFB laser) because the diffraction grating is formed to spatially modulate an equivalent refractive index distribution and to feed back the light in a distributed manner. Noted that only the main part of the laser is shown in FIGS. 1A and 1B, and elements such as electrodes and an anti-reflection coating film are not illustrated.
The structure in FIG. 1A has unique characteristics unlike a simple distributed-feedback semiconductor laser. A phase of the diffraction grating 5 is shifted by a half wavelength at a central portion of the cavity. A junction plane 4 a is formed such that the phase of light transmitted through the central portion of the cavity is shifted by a 1/4 wavelength since the phase of the equivalent refractive index distribution upon spatial modulation is shifted by a half wavelength, as is well known to those skilled in the art.
In a conventional DFB structure without the 1/4 wavelength phase shift, minimum external coupling loss occurs at two resonance frequency portions, which is shifted from the Bragg frequency f.sub.0 by a predetermined frequency (f.sub.1). In other words, two optical modes having frequencies f.sub.0 .+-.f.sub.1 are present. In the structure where the phase shift portion 4a shown in FIG. 1A is formed, the phase of a light component is shifted when it travels through the phase shift portion 4a. The number of resonance modes for giving the minimum external coupling loss is reduced to one, and its resonance frequency coincides with the Bragg frequency f.sub.0. Therefore, stable single mode oscillation can be achieved.
The above description is associated with the structure shown in FIG. 1A. It is known that a practical structure for shifting the optical phase by a 1/4 wavelength can be easily achieved in other structures. More specifically, by partially omitting the diffraction grating, the thickness (d1) of the active layer 1 under this portion (4b) can be made different from the thickness (d2) of the portion above which the diffraction grating is present. In addition, when the width W of the no diffraction grating portion is properly set by a relationship between the thicknesses d2 and d1, the same effect as in FIG. 1A can be obtained. That is, the phase of light propagating through the laser device can be shifted by a 1/4 wavelength through the portion (4b). Therefore, stable single mode oscillation can be achieved in the structure of FIG. 1B.
The phase-shift distributed-feedback semiconductor laser shown in FIGS. 1A and 1B is more effective than the simple distributed-feedback semiconductor laser (DFB) without a phase-shift as far as the mode property is improved from a plural to single mode oscillation. However, no consideration has been made to obtain a narrow spectral width of light oscillated in the single mode. This conventional structure, however, has a decisive drawback (to be described in detail below) in that the spectral width cannot be sufficiently narrowed even if structure parameters such as a cavity length are optimized.
A spectral line width .DELTA..nu. of the distributed-feedback (DFB) semiconductor laser using a diffraction grating is represented as follows: EQU .DELTA..nu.=(K/L)(.alpha..sub.0 +.alpha..sub.th L/L)(1/(J/J.sub.th -1)) (1)
where K is the constant depending primarily on semiconductor material parameters, L is the overall cavity length, .alpha..sub.0 and .alpha..sub.th are the internal cavity loss and the external coupling loss per unit length respectively, J is the injection current, and J.sub.th is the threshold current. Note that .alpha..sub.0 is the constant depending on the device fabrication conditions.
.alpha..sub.th L is a value depending on the structure and can be obtained as a function of L and a coupling constant .kappa., which corresponds to the feedback strength, by solving a coupled mode equation accompanied with boundary conditions, wherein the coupling constant .kappa. is a value proportional to the depth of the groove of the diffraction grating, i.e., the depth of spatial modulation of the equivalent refractive index.
The spectral width .DELTA..nu. of the DFB laser decreases, according to equation (1), when the cavity length L is increased, and the magnitude of the injection current J is set to be larger compared to the threshold current J.sub.th. The spatial hole burning effect are imposed as limitations on the spectral width of the DFB laser having the 1/4 wavelength phase-shift portion shown in FIGS. 1A and 1B. The light intensity distribution inside the cavity of this structure is known to have a shape strongly concentrated and confined at the center when the dimensionless feedback strength .kappa.L is large, as shown in FIG. 2.
The mode behavior such as the light intensity distribution is given as a function of a dimensionless feedback strength .kappa.L, i.e., the product of the constant representing the feedback strength and the cavity length L, because the coupled mode equation is kept constant with respect to L scale conversion, i.e., all structure and mode parameters can be normalized by the length so as to obtain dimensionless values.
When the feedback strength .kappa.L is large, the dimensionless external coupling loss .alpha..sub.th L can also be derived as follows: EQU .alpha..sub.th L=(.kappa.L/sinh(.kappa.L/2))(cosh(.kappa.L/2)-sinh(.kappa.L/2)) (2)
A substitution of equation (2) into equation (1) yields the relationship between the oscillation spectral width .DELTA..nu. and the cavity length L in the conventional phase shift distributed-feedback semiconductor laser shown in FIGS. 1A and 1B. .alpha..sub.th L is a decreasing function with respect to the feedback strength .kappa.L. Therefore, when the injection current J is kept constant, the oscillation spectral width .DELTA..nu. can be narrowed in accordance with an increase in the cavity length L, as indicated by a solid curve in FIG. 8.
However, it is impossible to keep the injection current constant without causing the instability of the laser oscillation when the cavity length L is extended, as will be described in the following. As described above, the structure shown in FIGS. 1A and 1B has the peaked light intensity distribution in the cavity, as shown in FIG. 2. When the feedback strength .kappa.L is increased, the distribution is strongly concentrated at the center. The injection current J is a function of the constant .kappa. and the overall cavity length L. Therefore, when the injection current J is increased, the number of carriers near the center of the cavity is decreased as compared with that near the end faces of the cavity in the light intensity distribution due to the reason described below. The magnitude of the stimulated emission is proportional to the light intensity and thus the stimulated emission is stronger where the light intensity is strong. When the stimulated emission is strong, more carriers are used for the stimulated emission. Thus, the number of carriers decreases at the point where the light intensity is strong.
The shape of the carrier distribution is a V-shaped distribution where the central portion is recessed, as opposed to an inverted V-shaped distribution of the light intensity where the central portion is the highest peak shown in FIG. 2. A phenomenon in which a shortage of carriers locally occurs is called spatial hole burning effect. In the present case, the spatial hole burning effect occurs along the axial direction of the cavity. Nonuniformity of the carrier distribution causes that of the refractive index distribution since the change in refractive index is proportional to the number of carriers. Nonuniformity of the refractive index distribution causes a change in light intensity distribution in the oscillation mode due to the change of the amount of the distributed feedback in the presence of the grating. As a result, for example, a mode of a higher order may be oscillated, and thus the oscillation becomes unstable.
Under the condition that the stable single mode oscillation is guaranteed, the oscillation spectral line width .DELTA..nu. is saturated even if the cavity length is increased, as indicated by a dotted line in FIG. 8. Therefore, this width cannot be further decreased, as indicated by a dotted line a in FIG. 8. The dotted line a in FIG. 8 indicates an analyzed result when the feedback strength .kappa.L is set to be 1.25 so as to optimally stabilize the oscillation mode in the conventional structure. In order to stabilize the mode in the conventional structure, a structure which has a light intensity distribution as flat as possible and can minimize hole burning, i.e., a structure having a feedback strength of around 1.25 is reported to be preferable (H. Soda et. al., Proc. I.E.E.E., Semiconductor Laser Conference, Kanazawa, 1986). In other words, under the condition of stable single mode oscillation, only the oscillation spectral width corresponding to about 1 MHz can be obtained in the conventional structure.
A conventional FM modulation scheme for a semiconductor laser is shown in FIG. 3A (S. Yamazaki et. al., Electron. Lett., Vol. 22, P. 5 (1986)). A constant current is supplied to a DFB laser I to cause oscillation in a stable mode. When an injection current supplied to a laser II is modulated around the value smaller than the threshold value, a reflectivity at the right facet of the laser I is effectively modulated by the refractive index modulation of the laser II, thereby causing FM modulation. According to this scheme, FM modulation strongly depends on the phase of the DFB grating at the facet, and therefore, modulation characteristics undesirably vary between the devices produced at a time.
Still another conventional example is shown in FIG. 3B. In a structure obtained by dividing an electrode of a DFB laser into 3 electrodes, different injection currents are applied to electrodes and an asymmetrical refractive index distribution can be obtained. Therefore, the oscillation mode can be changed (Y. Yoshikuni and G. Motosugi, Proc. Opt. Fiber Conf., Atlanta, 1986). When a bias current in good conditions is supplied to each electrode and modulation currents of opposite phases are applied to an end electrode and the central electrode, the DFB structure can be oscillated in the single mode to achieve FM modulation free from an AM component. According to this scheme, the oscillation frequency and the modulation characteristics are changed depending on operating conditions, and thus the conditions must be set upon at every operation. Long-term stability and uniformity of the device characteristics used at remote locations cannot be perfectly satisfied. Reference numerals and symbols in FIG. 3 correspond to those in FIG. 1, and reference symbols E.sub.a and E.sub.b denote electrodes.